1. Field of the Invention
This invention relates to a fiber optical network system, and more particularly to a signal switching module for optical network monitoring and fault locating, which is applied to a wavelength-division-multiplexing (WDM) passive optical network to effectively detect faults of the wavelength-division-multiplexing passive optical network.
2. Description of the Prior Art
Numerous wavelength-division-multiplexing technical schemes have been proposed to expand the capacity of a passive optical network. The attractive characteristics of a WDM-PON, such as high speed, high capacity, and a low bit-per-second price, allow such a network to support huge real-time on-demand data transmission from the Internet. To maintain a high reliability, a wavelength-division-multiplexing (WDM) passive optical network needs to have an efficient method to cover network status monitoring and response to failure conditions and emergency situations. A significant emphasis has been placed on maintaining and monitoring of a fiber communication network architecture for Internet applications. Network monitoring is important for maintaining a fiber optic communication network to detect and locate break points when failures occur in branch paths of the fiber communication network. Moreover, monitoring and fault-diagnosis methods need higher flexibility so that they can determine continuously real-time network status and detect break points when failures occur in the fiber communication network.
An optical time-domain reflectometer (OTDR) is an instrument that is used for evaluating the physical fidelity of a fiber link. In normal operation, the OTDR sends a series of pulse signals into a fiber network to detect what the fiber connection situation is. The detection mechanism for receiving the returning optical signal is at the same location as that for sending pulse signals, and the transmitted optical pulse signals are scattered and reflected back as they encounter different refractive index media along the transmission path. The reflected optical signal is then detected, and it is function of time which can be transformed to a function of distance along the fiber to identify and locate faults. Therefore, the OTDR may be used for measuring parameters such as fiber attenuation, fiber length, optical connector and splice losses, light reflection levels, or anomalies along the fiber path.
Numerous prior concepts have been proposed for the use of an OTDR to monitor each WDM channel independently. In these concepts, the OTDR is located at the central office (CO) and the OTDR signal passes through an arrayed waveguide grating (AWG) located at both the CO and the optical network unit (ONU) at the user location. The alternative in the broadband-OTDR case is to make each distribution fiber have a different length, but this is a cumbersome implementation strategy, referring to FTTx PON Technology and Testing, EXFO Inc., Quebec City, Canada, 2005, proposed by A. Girard.
Solutions based on tunable OTDR methods add cost and complexity due to the requirement of a tunable laser source at the CO and the need to schedule OTDR pulse transmission sequentially in a round-robin channel-by-channel manner among the branches of the WDM PON. The concept of Chen et al. (W. Chen, B. De Mulder, J. Vandewege, and X. Z. Qiu), entitled “Embedded OTDR Monitoring of the Fiber Plant behind the PON Power Splitter,” Proc. Symp. IEEE/LEOS Benelux Chapter, 2006, Eindhoven, pp. 13-16, requires embedding an OTDR function into each ONU transceiver, which increases the cost of the user device. The method of Hann et al. (S. Hann, J.-S. Yoo, and C.-S. Park,), entitled “Monitoring technique for a hybrid PS/WDM-PON by using a tunable OTDR and FBGs,” Meas. Sci. Technol., vol. 17, pp. 1070-1074, April 2006, is based on using a tunable OTDR in conjunction with an individual fiber Bragg grating (FBG) at each ONU, which requires a complex implementation strategy and is not easily expandable. Moreover, the monitoring schemes of Park et al. (J. Park, J. Baik, and C. Lee), entitled “Fault-detection technique in a WDM-PON,” Optics Express, vol. 15, pp. 1461-1466, 19 Feb. 2007, require a tunable OTDR, so they have limited expandability capability for adding more users. The proposal of Lim et al. (K. W. Lim, E. S. Son, K. H. Han, and Y. C. Chung), entitled “Fault Localization in WDM Passive Optical Network by Reusing Downstream Light Sources,” IEEE Photonics Technol. Letters, vol. 17, pp 2691-2693, December 2005, uses the optical transmitter at the CO to transmit an OTDR pulse upon detecting the absence of upstream signals, which again requires the use of a tunable OTDR to send specific wavelength signals to given failed branches.
Another class of monitoring methods combines the use of a broadband light source that is spectrally sliced at the remote node into multiple monitoring channels. Wavelength-dependent or spectral band-dependent optical reflectors at each ONU then reflect the monitoring channel back to the CO by means of an optical reflector of a wavelength-dependent component such as a fiber Bragg grating, referring to the proposal of S. B. Park, D. K. Jung, H. S. Shin, S. Hwang, Y. Oh, C. Shim, IEE Electron. Lett., vol 42, pp. 239-240, February 2006, or a combination of a wavelength coupler and a wideband mirror centered on the emission waveband of the broadband source, referring to the proposal of K. Lee, S. B. Kang, D. S. Lim, H. K. Lee, and W. V. Sorin, entitled “Fiber Link Loss Monitoring Scheme in Bidirectional WDM Transmission Using ASE-Injected FP-LD,” IEEE Photonics Technol. Letters, vol. 18, pp 523-525, 1 Feb. 2006. At the CO the channels are detected using either a series of optical power meters in the proposal of Park et al. or an optical spectrum analyzer (OSA) in the proposal of Lee et al. These methods are limited to detecting link loss and do not identify the location of the fault.
Besides, alternative approaches involve the use of optical encoding methods, proposed by H. Fathallah, M. M. Rad, and L. A. Rusch, entitled “PON Monitoring: Periodic Encoders with Low Capital and Operational Cost,” IEEE Photonics Technol. Letters, vol. 20, pp. 2039-2041, 15 Dec. 2008, implementation low-cost vertical cavity surface-emitting lasers (VCSELs) for channel monitoring, proposed by E. Wong, X. Chao, and C. J. Chang-Hasnain, entitled “Upstream vertical cavity surface-emitting lasers for fault monitoring and localization in WDM passive optical networks,” Optics Communications, vol. 281, pp. 2218-2226, 2008, or the design of a fail-safe architecture that can provide protection against fiber failures, proposed by A. Chowdhury, M.-F. Huang, H.-C. Chien, G. Ellinas, and G.-K. Chang, entitled “A Self-Survivable WDM-PON Architecture with Centralized Wavelength Monitoring, Protection and Restoration for both Upstream and Downstream Links,” OFC/NFOEC 2008 Conf. Proc., San Diego, Paper JThA95, proposed by T.-J. Chan, C.-K. Chan, L.-K. Chen, and F. Tong, entitled “A Self-Protected Architecture for Wavelength-Division-Multiplexed Passive Optical Networks,” IEEE Photonics Technol. Letters, vol. 15, pp 1660-1662, November 2003, proposed by K. Lee, S. B. Lee, J. H. Lee, Y.-G. Han, S.-G. Mun, S.-M. Lee, and C.-H. Lee, entitled “A self-restorable architecture for bidirectional wavelength-division-multiplexed passive optical network with colorless ONUs,” Optics Express, vol. 15, pp. 4863-4868, 16 Apr. 2007, and proposed by X. Cheng, Y. J. Wen, Z. Xu, Y. Wang, and Y.-K. Yeo, entitled “Survivable WDM-PON with self-protection and in-service fault localization capabilities,” Optics Communications, vol. 281, pp. 4606-4611, 2008. However, none of these methods are able to pinpoint the exact fault locations.
In view of the aforementioned drawbacks, the present invention provides an improved fault monitoring system for real-time monitoring nodes and simultaneously determining a fault location.